Stoichiometric Single Crystals Research Articles

Vibrations of H+(D+) in stoichiometric LiNbO3 single crystal

A first principles quantum mechanical calculation of the vibrational energy levels and transition frequencies associated with protons in stoichiometric LiNbO(3) single crystal has been carried out. The hydrogen contaminated crystal has been approximated by a model one obtains by translating a supercell, i.e., a cluster of LiNbO(3) unit cells containing a single H(+) and a Li(+) vacancy. Based on the supercell model an approximate Hamiltonian operator describing vibrations of the proton sublattice embedded in the host crystal has been derived. It is further simplified to a sum of uncoupled Hamiltonian operators corresponding to different wave vectors (ks) and each describing vibrations of a quasi-particle (quasi-proton). The three dimensional (3D) Hamiltonian operator of k=0 has been employed to calculate vibrational levels and transition frequencies. The potential energy surface (PES) entering this Hamiltonian operator has been calculated point wise on a large set of grid points by using density functional theory, and an analytical approximation to the PES has been constructed by non-parametric approximation. Then, the nuclear motion Schrödinger equation has been solved by employing the method of discrete variable representation. It has been found that the (quasi-)H(+) vibrates in a strongly anharmonic PES. Its vibrations can be described approximately as a stretching, and two orthogonal bending vibrations. The theoretically calculated transition frequencies agree within 1% with those experimentally determined, and they have allowed the assignment of one of the hitherto unassigned bands as a combination of the stretching and the bending of lower fundamental frequency.

Combination transitions due to stretching and librations of OH^− ions in LiNbO_3

A new absorption band has been detected at 4009 cm⁻¹ in stoichiometric LiNbO₃ single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamental frequency than that contributing to the combination band at 4415 cm¹ observed by Gröne and Kapphan [J. Phys. Condens. Matter 7, 3051 (1995)].

Compression of Micropillars of TiAl Coexisting with Ti3Al

ABSTRACTSquare-shaped micropillar of nearly stoichiometric TiAl single crystals with various loading axis orientations were prepared from TiAl PST crystals by focused ion beam (FIB) technique and deformed in compression using a micro hardness testing machine equipped with a flat diamond tip in order to investigate the values of CRSS for the three types of operative deformation modes, namely ${111}1/2< 1 \bar 10 >$ ordinary slip, ${111}< \bar 101 >$ superlattice slip and ${111}<11\bar 2 >$ twinning. The selective activation of the three types of deformation modes was confirmed to be achieved by compression tests of $[\bar 2 11]-, \bar 110- and [\bar 2 12]$-oriented single crystalline micropillars, respectively. The average CRSS values for ordinary slip, superlattice slip and deformation twinning obtained for micropillars with an initial side length between 3.8 and 7.9μm were estimated to be about 145, 284 and 113 MPa, respectively.

Kinetics of Photorefractive Light Scattering in Stoichiometric LiNbO3 Single Crystals Grown from Melt Containing 58.6 Mole% of Li2O

A study of kinetics of photo-refractive light scattering in ostensibly pure stoichiometric LiNbO3 (Li/Nb = 1) single crystal grown from melt containing 58.6 mole% Li2O excited by laser radiation of 0.53 μm is reported. Asymmetry of the distribution of scattered light intensity is revealed and found to be a linear function of the intensity of the exciting radiation in the 35–160 mW range. Due to heating of the crystal, at farther increase of the radiation intensity the distribution of scattered light contracts.

Quasi-Two-Dimensional Fermi-Liquid State in Sr2RhO4-δ

Single crystals of layered perovskite Sr 2 RhO 4-δ (δ=0.0 and 0.1) are successfully grown by the floating-zone method. Stoichiometric single crystals (Sr 2 RhO 4.0 ) are obtained by O 2 -annealing the as-grown crystals (Sr 2 RhO 3.9 ). Sr 2 RhO 4.0 and Sr 2 RhO 3.9 show quasi-two-dimensional Fermi-liquid behavior at low temperatures, whereas there are large differences in the anisotropy of electrical resistivity ρ c (3 K)/ρ a b (3 K) and Wilson ratio R w between Sr 2 RhO 4.0 and Sr 2 RhO 3.9 : ρ c (3 K)/ρ a b (3 K) = 2400 (19000) and R w =3.8 (6.4) for Sr 2 RhO 4.0 (Sr 2 RhO 3.9 ). The differences observed between the temperature dependence of the in-plane electrical resistivity ( T <15 K), magnetic susceptibility ( T <300 K) and specific heat ( T <10 K) of Sr 2 RhO 4.0 and Sr 2 RhO 3.9 are mainly derived from those between the density of states and band structure near the corresponding Fermi level. This indicates that the changes in these physical properties, which are accompanied by oxygen defects in th...

Photorefractive Properties of Stoichiometric Lithium Niobate Single Crystals

The specific features of photorefractive light scattering in nominally pure stoichiometric (Li/Nb = 1) sin- gle crystals grown from a melt with 58.6 mol % Li2O (LiNbO3st) and in the stoichiometric single crystals grown from a melt of congruent composition in the presence of K2O flux (LiNbO3stK2O) have been investi- gated. At an excitation power of 30 mW, LiNbO3stK2O single crystals are found to exhibit a stronger photo- refractive effect than LiNbO3st single crystals.

Mössbauer study on the polar ferrimagnet GaFeO3

In order to clarify the origin of magnetoelectric (ME) effect in the polar ferrimagnet GaFeO3, we have conducted Mössbauer spectroscopy by using a stoichiometric single crystal. The spectra are composed of three subspectra, corresponding to Fe3+ ions on Fe1, Fe2, and Ga2 sites. The quadrupole splitting (QS) of Fe1 subspectrum, 1.05mm/s, is too large to be explained by the electric field gradients (EFG) due to the lattice contribution. At 5K, there is no difference in the quadrupole parameters between the spectrum with and without applied magnetic field, suggesting that the ME effect is not caused by a displacement of ions. Note that in applied magnetic field, Fe1 subspectrum shows extra increase in observed field of about 10kOe. In conjunction with the large QS, this feature may be due to the spin-orbit interaction, which can also be the cause of the large magnetic anisotropy.

LiNbO3섬유 단결정의 분극에 관한 연구

Congruent or stoichiometric LiNbO₃ fiber single crystals were grown by the μ-PD method, and the grown fiber crystals have the several (2 or 3) ridges with a diameter of 1.35~1.5 ㎜ and a length of 40~100 ㎜. In this μ-PD process, different growth rates (10~60 ㎜/h) were applied. Pt wire or LiNbO₃ crystal was used as a seed. The properties of grown LiNbO₃ fiber single crystals having a-axis or c-axis according to seeds were effected by the grown conditions(Pt tube diameter, pulling speed, after heater etc.). Disk-type LiNbO₃ samples were poled in condition of DC 5 V/㎝ at 1050, 1075 or 1100℃. XRD, SEM, conoscope image through the polarized microscope, TC measuring apparatus, optical transmittance measuring instrument are used to identify the properties of LiNbO₃.

Ternary system Li 2O–K 2O–Nb 2O 5: Re-examination of the 30 mol% K 2O isopleth and growth of fully stoichiometric potassium lithium niobate single crystals by the micro-pulling down technique

Potassium lithium niobate (KLN) single crystals have attractive properties for non-linear optical applications based on frequency conversion of laser diodes in the blue range. Especially, fully stoichiometric K 3Li 2Nb 5O 15 crystals would be capable of doubling a laser light in the near UV range. Using powder X-ray diffraction and DSC experiments, we have re-investigated the 30 mol% K 2O isopleth of the ternary system Li 2O–K 2O–Nb 2O 5 in order to explore the possibility of a limited existence field for this phase. From our results, it was shown that the stoichiometric KLN phase exists between 970 and 1040 °C, temperature at which it undergoes a non-congruent melting. From this conclusion, compositionally homogeneous a-axis oriented single crystals fibers of stoichiometric K 3Li 2Nb 5O 15 were successfully grown by the micro-pulling down technique with pulling rates in the range 0.3–0.7 mm min −1. The crystal length was between 10 and 120 mm for an apparent diameter near 500 μm. The fibers, characterized by optical microscopy, X-ray diffraction and Raman spectroscopy, appeared free of macro-defects and of good quality and their stoichiometric composition was also confirmed.

Substitution effect on the thermoelectric properties of reduced Nb-doped Sr 0.95La 0.05TiO 3 ceramics

Ceramic samples of Sr 0.95La 0.05Ti 1− x Nb x O 3 (0 ≤ x ≤ 0.10) have been prepared by the solid-state reaction method. From their crystal structures determined by means of the powder X-ray diffraction, we found that the lattice parameters increase with increasing niobium content. The electrical resistivity, Seebeck coefficient and thermal conductivity have been measured in the temperature range between 300 K and 1100 K. All samples show large negative Seebeck coefficient, which is indicative of a n-type conduction. The high-temperature electrical resistivity and the absolute Seebeck coefficients decrease with the increase of niobium content. The electrical resistivity decreases from 10.3 mΩ cm for x = 0 at 1043 K to 5.4 mΩ cm for x = 0.10 at 1042 K. The thermal conductivity decreases with increasing temperature for all samples, and shows no distinct influence by the niobium substitution. The power factor is decreased in niobium doping samples, as there is no effectively decreasing of the electrical resistivity but reduction of Seebeck coefficient. Therefore, the highest thermoelectric figure of merit still observed in Sr 0.95La 0.05TiO 3, reaches 0.21 at 973 K, which is 40% larger than that of the stoichiometric single crystal.

Evaluation of anisotropy of critical current density in stoichiometric Bi-2212 single crystals

The anisotropy of the critical current density was estimated for stoichiometric Bi 2 Sr 2 CaCu 2 O y (Bi-2212) and Bi 2.1 Sr 1.8 Ca 1.0 Cu 2.0 O y single crystals. For this purpose, the critical current density in the a– b plane J c ab and that along the c-axis J c c were discriminated from measurement of DC magnetic moments for specimens with different aspect ratio. The magnetic moment of specimen was measured by a SQUID magnetometer in a magnetic field parallel to the wide surface which is normal to the c-axis. Both of J c ab and J c c are larger in the stoichiometric specimen. In addition, the anisotropy, J c ab / J c c , is found to be significantly reduced in the stoichiometric specimen.

ChemInform Abstract: Stoichiometric Single Crystal Growth of AgGaS2 by Iodine Transport Method and Characterization.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Frequency and field dependent susceptibility of magnetite at low temperature

Abstract We report the temperature dependence of in-phase and quadrature susceptibilities, k′ and k″, between 20 K and 300 K for a stoichiometric natural single crystal of magnetite. Measurements were made for amplitudes of the AC driving field ranging from H = 30 A/m to 2 kA/m and frequencies ranging from f = 40 Hz to 4 kHz. In cubic magnetite above the Verwey transition, TV = 120 K, k′ is limited by self-demagnetization and does not vary greatly with T, H or f. As the crystal cools through TV and transforms to monoclinic structure, k′ decreases by about a factor 2, with a further more gradual decrease of 10-20% in cooling from 40 to 20 K. Saturation remanence also drops sharply at T V but shows no further change in cooling below 40 K. Thus it appears that domain walls remain pinned throughout the 20-40 K range but small segments undergo reversible oscillations in an AC field, the amplitude of oscillation decreasing steadily with cooling below 40 K. In this same range, k″ reaches a peak, while the temperature at which k′ decreases most rapidly changes with frequency. Both observations indicate that domain wall oscillations lag appreciably behind the driving field at very low temperature. Both k′ and k″ increase markedly with increasing AC field amplitude below TV. The field dependence is particularly strong below 40 K. Analysis of the k′(f) data between 20 and 40 K based on an Arrhenius thermal activation equation gives a pre-exponential frequency factor f o ≈ 2.5 × 108 s-1 and an activation energy ΔE = 0.035 eV. The ΔE is appropriate for electron hopping but f o suggests an indirect mechanism for wall mobility related to changes in electron ordering within walls.

Manifestation of the specific structural features of lithium niobate single crystals of different composition in their Raman spectra

The Raman spectra of nominally pure lithium niobate single crystals of congruent, close to stoichiometric, and stoichiometric compositions and congruent lithium niobate single crystals doped with Gd3+, Y3+, and Mg2+ ions have been investigated. Weak lines whose widths anomalously decrease with an increase in cation sublattice disordering at a change in the single crystal composition were found for the first time. These lines may be indicative of fine ordering processes involving structural units of the cation sublattice, as a result of which this sublattice is disordered as a whole.

An optical spectroscopy study of defects in lithium tantalate single crystals

The congruent, stoichiometric, and Mg doped stoichiometric LiTaO3 single crystals have been successfully grown by the Czochralski technique. The evolution of defect structures caused by varying composition and post-growth processing has been evaluated from the optical absorption and photoluminescence measurements. Optical absorption studies showed that the UV absorption edge is very sensitive to the composition of LiTaO3 crystals. Photoluminescence of various LiTaO3 single crystals at room temperature was observed. The emission bands centered at 360, 430, and 530nm were assigned to different defects, which can well show the defect information in LiTaO3 crystals.

Plastic Deformation Behavior of Ti5Si3 Single Crystals

AbstractDeformation behavior of binary stoichiometric Ti5Si3 single crystals was examined as a function of the loading axis orientation and temperature. Two different types of deformation modes, namely {1100}[0001] prism slip, {2112}1/3&lt;2113&gt; pyramidal slip were newly identified to be activated above 1300 °C depending on the loading axis orientation. Critical resolved shear stresses (CRSS) for the {1100}[0001] prism slip and {2112}1/3&lt;2113&gt; pyramidal slip were estimated to be about 130 MPa and 330 MPa at 1400 °C, respectively. The values of the CRSS for these two slip systems decrease monotonously with increasing the temperature.

Spectroscopic and magnetic susceptibility analyses of the 7F J and 5D 4 energy levels of Tb 3+(4f 8) in TbAlO 3

Detailed analyses of spectroscopic and temperature-dependent magnetic susceptibility data are reported for the crystal-field split energy levels of the 7F J and 5D 4 of Tb 3+ in stoichiometric single crystals of ortho-aluminate TbAlO 3. The spectroscopic data include absorption spectra obtained between 2940 and 480 nm from 8 to 300 K. High resolution fluorescence spectra are reported, representing transitions from 5D 4 to 7F 6,5,4, at a sample temperature of 85 K. Using crystal-field modeling techniques recently adapted for low symmetry systems, we have assigned all 58 experimental Stark levels within the 7F J and 5D 4 manifolds, with a fitting standard deviation of 4.5 cm −1 (3.8 cm −1 rms error). As a further test, the theoretical Stark levels and calculated wavefunctions were used to determine the temperature dependence of the magnetic susceptibility along the c-axis of the TbAlO 3 crystal. Good agreement is obtained between the calculated susceptibility and temperature-dependent magnetic data reported earlier, including a prediction of a 0.2 cm −1 splitting of the ground-state quasi-doublet. The susceptibility calculation also confirms the predicted ordering of states within the 7F 6 multiplet manifold.

An athermal deformation model of the yield stress anomaly in Ni 3Al

An athermal deformation model was proposed for the yield stress anomaly in Ni 3Al based on the concept that dislocation multiplication versus the immobilization by the Kear–Wilsdorf (KW) locking controls the plastic deformation. In the model dislocations, superkinks lying between two KW locks multiply through expansion in an athermal manner. Regarding the KW locking, the entire screw segment cross-slips onto (010) to form long KW locks in one thermal activation event. The model can explain the macroscopic characteristics of the deformation, the strain-rate independent of the yield stress and the compliance with Schmid's law, which were recently revealed in binary, stoichiometric Ni 3Al single crystals. Quantitative assessment proved that the model reproduces the temperature dependence of the yield stress with reasonable fitting parameters. The driving force for the KW locking, one of the most important parameters for the yield stress anomaly, was evaluated at 127 mJ m −2. This value is in good agreement with those calculated from the anti-phase boundary energies on {111} and {100} reported in the literature.

Magnetic properties of Bi2212 single crystals with Bi:Sr:Ca:Cu = 2:2:1:2

Bi2212 single crystals having various cation compositions were grown by the floating zone method and evaluated their magnetic properties and resistivity after control of oxygen content by post-annealing. The in-plane anisotropy in the normal state resistivity of Bi2212 single crystals systematically increased with approaching cation compositions to be stoichiometric, 2:2:1:2. This means that disorders in modulation structure due to cation nonstoichiometry strongly affects the in-plane electronic anisotropy of Bi2212. On the magnetic phase diagram under H// c, large differences were not found in the second peak behavior and first order transition (FOT) line of the vortex system between the cation stoichiometric and nonstoichiometric single crystals, while strong bulk pinning was effective up to the FOT line for the former crystals with their irreversibility lines just on the FOT line. In addition, the entropy change of the vortex at the FOT of cation stoichiometric crystals was apparently larger than that of conventional ones. These suggest that more homogeneous and strong superconductivity occurs in the cation stoichiometric “Bi2212” crystals.

Growth and characterization of near stoichiometric LiNbO3 single crystal

AbstractA near stoichiometric LiNbO3 single crystal has been grown by the Czochralski method from a 58.5% Li melt hold in a large platinum crucible. High resolution X‐ray rocking curves of 30$ \bar 3 $0 and 0006 reflections indicated that the near stoichiometric LiNbO3 crystal possesses the high structural quality. Compared with the congruent LiNbO3, the near stoichiometric LiNbO3 possesses shorter ultraviolet absorption edge, thus higher Li concentration. The OH– infrared absorption band analyses showed that the Li concentration in the near stoichiometric LiNbO3 crystal is higher than that in the congruent LiNbO3 crystal. This result is in good agreement with that of the ultraviolet absorption edge. The electro‐optic (EO) coefficient γ22 of the near stoichiometric LiNbO3 crystal was measured to be 6.75 pm/V higher than that of congruent LiNbO3 crystal. It also proves the near stoichiometric LiNbO3 electro‐optic Q‐switched requires a low driving voltage and it is advantageous for the device performance. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)